When we have 2 different ISP;s, I would suggest for more controlled and filtered way for sending the traffic from 2 different ISP's.

I would suggest to use NAT and PBR (policy based routing).I would first look at the kind of traffic, users, applications that I have to go the outisde world. Once the traffic is determined I would see the traffic distribution of the users over the two links. I would use my application, mail traffic to go thorough one link and internet and other traffic to use another link.I will make both the links as back up of each other.In case of one of the link goes my all the traffic will use the other link. I would suggest to implement proper QOS and rate limiting to drop the unwanted traffic.